1. Field of the Invention
The present invention relates to a laser apparatus and a laser irradiation method, specially a laser apparatus and a laser irradiation method that have a function of stabilizing energy of an output laser beam. The present invention also relates to a manufacturing method of a semiconductor device including a step for crystallizing a semiconductor film with the laser apparatus and the laser irradiation method.
2. Description of the Related Arts
Lasers are classified into a gas laser and a solid laser according to a laser medium, and have various kinds. In lasers, obtained wavelengths, energy values and pulse characteristics are different depending on the laser mediums, and applications of lasers have been searched according to characteristics of laser beams. Of various lasers, YAG laser, CO2 laser, excimer laser and the like are mainly used for many industrial laser apparatuses.
An excimer laser that is a gas laser emits intensive ultraviolet light and has a short wavelength of 0.193 μm (ArF) or 0.351 μm (XeF), and is superior in light condensing. Therefore, the excimer laser is suitable for a field in which a super microfabrication at μm level is required as typified by semiconductor manufacturing including mask forming, in addition to general processing of parts.
In a gas laser typified by an excimer laser or a CO2 laser, however, fluctuation in pressure of gas that is a laser medium is easily generated in an oscillator generally, and fluctuation in gas flow rate is also easily generated in the case of circulating gas of a laser medium in an electric discharge tube for oscillation. Consequently, there is a problem that it is difficult to stabilize energy of an output laser beam from the oscillator and to perform a process to an object uniformly.
Conventionally, fluctuation in pressure of a laser medium in a electric discharge tube is detected and another fluctuation in pressure for counteracting the fluctuation is generated to suppress the fluctuation in pressure or fluctuation in flow rate of the laser medium, in order to stabilize energy of an output laser beam (Reference 1: Japanese Patent Laid-Open No. Hei 07-038180).
The above reference 1 describes the following: fluctuation in pressure of gas that is laser medium is detected in a pressure sensor provided in a gas supplying duct and is input to a feedback control unit as a detection signal. A phase of the detection signal is inverted in the feedback control unit and the detection signal with the inverted phase is subjected to a frequency analysis with fast Fourier transform (FFT) in a waveform generator to obtain a waveform signal. After the waveform signal is amplified, it is input to a wave transducer. The wave transducer generates fluctuation in pressure, which has the inverted phase of and the same amplitude as that of the fluctuation in pressure detected by the pressure sensor, in accordance with the input amplified waveform to counteract the fluctuation in pressure of the laser medium.